Hi Dave,
You wrote:
Basically, as the antenna is set up, it appears to be a vertical elongated
umbrella structure with the eight downwires or 'top loading' wires extended
down nearly to ground level, quite close in to the mast proper. The reasons
for this were explained on the reflector some time back.
Perhaps I missed it, but I would be very interested to know why the umbrella
ribs are extended so far. My calculations show that the optimum length of
the ribs from the top is h(sqrt(1+n.sec(alpha))-1)/n, where h is the height
of the mast and alpha is the angle of the ribs from the vertical, and n is
the number of ribs, which for 10 degrees and 10 ribs (judging from the photo
of the mast) is approx 23%. By optimum I mean the length which makes the
Radiation Resistance a maximum. Of course that may not be the objective
here.
73s John, G4CNN
_______________________________________________________
Get 100% FREE Internet Access powered by Excite
Visit http://freelane.excite.com/freeisp
| 